Multiplex carrier-current signaling



Sept. 18, 1928.

H. NYQUIST MULTIPLEX CARRIER CURRENT SIGNALING Eiled May 13, 1926 3 Sh ets-Sheet H n n H W W FMWJMJ A TTOR/fli Y Sept. 18, 1928.

H. NYQUIST MULTIPLEX CARRIER CURRENT SIGNALING L 2 32 3 Sheets-Sheet 2 PR,

A TTOIINEV Patented Sept. 18, 1928.

UNITED STATES PATENT OFFICE.

HARRY NYQUIST, OF MILLBURN, NEVJ JERSEY, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY. A CORPORATION OF NEW YORK.

MULTIPLEX CARRIER-CURRENT SIGNALING.

Application filea May 13 An object of my invention is to provide a new and improved method and corresponding apparatus for multiplex signaling over a limited range or band of carrier current frequencies. Another object of my invention is to utilize such a band of frequencies advantageously for telegraph signal channels. Other objects of my invention relate to automatic adjustment to compensate for changes of transmission equivalent; generation at the receiving end of a current of the same frequency as the carrier current or currents, these currents being suppressed in transmission; and distinguishing message channels by phase and magnitude discrimination within the same frequency range.

In one aspect. my invention involves mul- [iplex transmission by the use of two stages of carrier current. one of intermediate frequency and the other of relatively h h frequency. There are various diiierent nels ailorded by dilierent neighboring: frequency hands on the intermediate frequency stage. and there is a further multiplication of channels by phase and magnitude discrimination on each of these intermediate frequencies. The invention relates largely to deriving. the proper intermediate frequency currents all of exact frequency value and in proper phase relation at the receiving end from the high frequency currents that are received there.

These and other objects of my invention will become apparent on consideration of an example of practice according to the invention which I will disclose in this specification together with a few modifications of certain features. It wil; be understood that the following disclosure relates to these particular examples of practice according to my invention and that the invention will be defined in the appended claims.

Referring); to the drawings Figure l a diagram of a radio transmitting! station; Fig. 2 is a diagram of the corresponding radio receiving station; Fig. 3 is a diagram of receiving apparatus and si;5'Ii:il-c mipensating apparatu shown symbolically n Fir 2: Figs. 4t, 5 and (i are ll2 f .Dl of alternative forms of synchroniziIre or frequencydetermining' apparatus shown symbolically in Fig. 2: Fig. 7 is a diagram of rec iv ng apparatus alternative to that snown in he upper part of Fig. 3; and Figs. 8 and 9 1926. @erial No. 1083-39.

are curve diaeranr: that will be referred to in explain ng Fig. 7.

Referring to Fig. 1. the constant speed motor M drives the generators G G... etc.. to produce currents of respective frequencies such. for examples. as A. mA and ,'i:'\. when m and 01- are odd integers. The output circuit of the generator G. ccinprises resistance and reactant-e elements having their related values indicated bv the leiends in Fig}; l, 9" being a certain resistance value. and 7' standing: for l. l follows that the electromotive forces applied through the transformer primaries 37 and 38 are 90 degrees apart in phase. The current of one of these ha es is contrtvlled hv the two keys K, and K The key IQ operates to reverse the phase of the component current put through the filter from the transformer primary winding 37; this is done without changing" the magnitude of this current. ()n the other her the kev K. operates to clu n c'e the man. do of this component of currcnt w i :tlz ut allot-ting its phase: when key K clo ed down or marking. the magn tude about three times what it is when this key open or spacing.

Similarly. the keys K and K respectivereverse the phase and change the magniude of the component of current put hrouqh the filter F. from th trans't'oriiier prin'iary winding 3 The liltcr F is a band pass filter of intermedia e frequency. the ame as the frequency of the generator (i The sending network that has inst been described is some hat sim ar t the send ng network shown in Fig. 1 of mv Patent 1.626. 7 5. granted March 1927 and fully described in the specification of that patent.

The boxes S, and 3 represent other sendingr networks corresponding to the sending network shown in the upper part of Fig. 1, and the filters F and F are band pass filters of intermed ate trcwicncies correspoiuling. snectively. to the nerz -tors G. and G The three or curr 'nts from the enerators G G etc. each decompos d into two components 90 degrees apart. and each component varied by phase reversals and by uiac nitiule chances for respective message sirrnaliner. ire all superposed in one pair of conductors and put th ouejh the amplifier A A balanced modulato is shown at 22. comprising; two three-electrode vacuum tubes in push-pull relation. The generator or oscillator 0 of radio. frequency. B furnishes-the carrier current input for thebalanced-moduand the superposed currents from Since the modulator 22 is balanced, the

carrier current is suppressed from its output.

and only the upper and lowersidebands are present. The hand filter F passes only the upper side band of frequencies B+A, B+3A. B+5A, etc. The currents of these frequencies are put on the antenna; 41; and radio waves of corresponding frequencies are transmitted from the antenna 41.

Referring to Fig. 2, theseradiowaves. are receivedon the loop aerial 42. and the corresponding currents generated therein are amplified in the amplifier A and-passed through the adjustable: artificial line AL and the demodulator 101' and the filter F to the transformer 128. This transformer. is conncctedon the input side of theamplifien 129 whose output goes 'throughthe transformer 130 to the hushars 102.

The demodulator 101 is also supplied'with. high frequencyv current from: an oscillator 0 governed in a manner presently to he described, so that its frequency is'lteptaccurately atthe value Bi. For theipresent, let it he assumed that the: frequency of the 0scillator O is C, a value nearly equal to B, and it will be shown in due course how regulation is-ell'ected-so that C iszmade accurately equal to B.

The output currentfrom the demodulator 101 rompr'ses two side bands lying, respectively, far above and far-belowitheneighboring fnequencies- C and B+-A, Bl SA, etc. The hand filter F passesonly the lower of these side handsso that the currentsputon the conductors 102. are of the frequencies Bi-Xvt). li+3A- C, B+5A-C, etc.

Relying on the assumption already stated that the atljustn'icnt of the oscillator 0 is such as to make B. and. C nearly equal, it will be seen that the currents. refcrred. to have nearly the frequencies A, 3A, 5A. etc. Thes con'ipouents. are separated into respective branch circuits by the band: filters such as. F], F F and-F and these components of the. respective frequencies go to the corresponding receiving networks such as R R R, and R One of these receiving networks R is shown in the upper part of. Fig.3. A. similarreceivingnetwork.is shown in Fig. 2 of my Patent 1.620.335, granted March 15, 1927, and fully des ribed in the specification of that patent. The input current onthe conductors- 108 Is some odd multiple. frequency ot'A. say out (assuming, as will be established presently, that B G). lttwill be rememhered that the'ciu'rent of-this frequency comprises two components 90 degrees apart, each affected by phase reversals and magnitude-changes. This current goes through the transformer whose secondary 5.357 has an intermediate tap at .50. The. flllloutput voltage. from this. transformer goes to the splitterhy which two 9U-dcgree components. are separated, and it .s one of these that: is-

superposed. on the inputs for the detectors D anvil) The generator 6:, is of frequency MA, as determined by means and method presently to he described, (and subject' to the provisional assumption already. stated, that: E C). and the phase of the component applied to the detectors D,.and D isthe same as for one of the two components coming in on the conductors 108, provided the key K, for that component at thesendingstation in its markingposition. Since the input electromotive forces to these detectors from the local sources are in phase With one component and in quadrature with the other. the detectors will. give no response for'the other component but only for. the one. component for which there .is phase agreement. With the keys K, and K closed down, both detectors .D, and D will give: substantial outputcurrent-s energizing, res 'iectively, the polar relay PR, and the neutral. relay NR and holding the r. armatures at marking position.

The-relay NR is marginal, and as soon as the key K is opened at the sending station and the magnitude of the corresponding component of cu :rent isdccreased, the relay NR will drop its armature to the spacing positionindependently oi whether the phase of the component is the same for the local.-

sourcc, or reversed. The armature of relay NR is sloW acting so t does not drop fora reversal of magnetism in the relay core.

On the other hand. the polar relay PR, and associated parts are adjusted that its core would be dcencrgized if subject to the local source, alone; aceordingly its operation is dependent only on the phase of the superposed incomingreieived voltage; and when the resultant'voltage to the detector D,C()111 prises two components in phase, the-relay PR, will be operated marking position, but when the incoming voltage comprises a component opposite in phase to the local source. the relay PR, will reverse and be at spacing position. Aco'irdingly. raising. the key K, at the sending end causes the armature of relay PR, to reverse. irrespective of the i'nagnitude of the current component in volved.

The foregoing description for the operation of detectors D and D for one of the 90-degree components on conductors 106 applies similarly to the rema ning detectors l) and D and their associated relays PR and NR which operate on the other 90-degree component on the condu tors 108.

Having described the receivers shown symbolically as at B in Fig. 2 and in detail in Fig. 3, attention will now be given to the matter of determining the frequency of the oscillator 0 in Fig. 2. iridged across the output conductors from the filter F band filter F which passes the bane of frequencies on the carrier of the same intermediate frequency mA as passed by hand filter F The output from filter F goes through an adjustable delaynetwork Y This network Y is merely a transducer that gives an output corresponding to its input but With a time delay or lag according to its adjustment. The output goes to the sin;- nal compensating network X shown in detail at the lower part of Fig. 3. where its structure is apparent without the necessity for verbal exposition at this point.

The currents at the input of X from Y. are of the approximate frequency 471A, but having each 90-degree component affected by manitude changes and by phase revers Each of these magnitude changes and each of these phase reversals determines the position of the corresponding relay armature 90, 91. 92, or 93 in the receiving network R and the positions of these armatures determine the energization of the respective maginets 94, 95, 90 and 97, in the network X The armatures of these last mentioned mam nets shift to change connections within the network IQ so as to compensate for the phase and magnitude changes referred to.

For example, if the key IL at the sending end is raised from its marking to its spacing position this will reverse the phase of one component of the current on the conductors 108 and Will cause the relay P3 to drop its armatures, thus deenereiziugr the magnet 94: and causing; it to drop its armatures. This Will. change the connections Within the network X so that the output on the conductors 109 will he the after as before the phase reversal of the Md one component assuming that de ay in Y. is so adjusted that the change in the cuirent from Y to X is held up un il relax: at operates. Similarly, a phase change in the other component of the current on co; "um tors 108 Will be compensated in the network X and magnitude cha of these components Will be compensated in the netwo k X so that the output on the conductors 109 will be a pure sine-Wave current without phase reversals of a component and w thout magnitude changes of component.

The output from the network X of Fig.

2 on the conductors 109 will be a current of the frequency B+mA.C and this Will go to the frequency multiplier FM Where its frequency will be multiplied by the factor 11., MA being the intermediate. frequency for the band pass lilter F Accordingly, the output from the multiplier FM will be a current. or the frequency vilH- nuh-k 'htl.

The frequency of the current passed by the hand liltcr F to the receiving network 12 is 15+ HA-U By a branch circuit, current of this frequency is sent through a delay network Y and si nal compensator X to the frequency multiplier FM where its frequency is multiplied by the factor on, giving an output of frequency ml3+mnA-mQ 'lhese two output currents from the frequency multipliers Fill and FM go to the apparatus shown symbolically at (a). in Fig. :2 and in alternative detail in Figs. 4, 5 and 0. Q, is a device which operates to inrrc or decrease the natural frequency of the connected oscillator 0 according to departures from equality of frequency of the two input curicnts on the respective conductor pairs 110 and 111. C uinection hetweci: Q and (9 is made by the conductor pair 112.

lef to Fig. 109 and 171 are,

respectively the inductance and the capacity of the frequcncyleterminin; circuit of the c cillator (V) in Fig. 2. In shunt to the condenser 171 is another condenser 117, and they are so adjusted that. the frequency of the r-;cillator is a little too high with condenser 1271 alone in circuit, and a little too low with condenser 11} in parallel. The parallel hranch for condenser 11'? is controlled by the armature 110 of a polarized \lithin apparatus (i in Fig. 4 two threw clect ode "acuun'i tuhes 114 and 115 are arianc'cd as shown with the re pective inputs froin the conductor pairs 110 and 111. Ac-

whcn the currents in 110 and Ill in quadrature, the output currents will balanced and the armature 116 will he open. in departure from the quadrature relation for the currents in 110 and 111 will be in the sen e to close the circuit of the '1 and this action keeps frein circuits 110 and 111.

i; HP) mm A C tie two frequency multii. a Fi l it will, he seen that equal if the frequency C of the 0'- new d1 equal to B. Accordingly, z d so that with th equencydetermining theie are cillalor (i lhe osc wi'idrnser circu t. and frucncy of o i creator than P). hi and condenser 7 in paral el, the frequen -1' of oscillator 0 will be slightly less than Itiwillg also be seen that any. departure from cqualityiof'B and G causes a corresponding departuretafromi equality of the output frequenciesm'B mnA- mC and 11.13 mnA nC from. the two frequency multipliers FM outputs of the two frequency multipliers,

andntheir departure from the quadrature relation effects a compensating adjustment inthe oscillaton O sothat its frequency is held accurately at the value B.

By the control of the oscillator 0 as heretofore described, the output of the signal compensator X ,is held accurately at frequency mA and the current of this fre* quency goes by a branch circuit through amplifier A and filter F" to drive the synchronous motor SM which in turn drives the generators G G etc..at frequencies corresponding respectively to G G etc. at thesending end. The output currents of these frequencies go through the adjustable phase shifters P P etc. to the receiving networks R IR ,.etc. and are there applied as described heretofore in connection with Fig.3.

The apparatus Q, of Fig. 2 may be energized tocontrol the frequency of the oscillator O in another way-by throwing the switches 105, 106 and 107. This cuts out the two frequency multipliers Flif and FM. and

' puts the current of frequency B+nAC on theinput conductor pair 111 to the device Q. The current to the synchronous motor SM will be of fre%1ency-B+mAC, and

thegenerators G etc. are arranged so that their frequencies are all harmonics of the fundamental having the frequency Z/m (B+mAC) and accordingly the frequency of generator G is n/m (B +mA C). The current of this frequency goes through switches 105 and through theinput conductor pair 110 to device, Q. Any departure from equality of these two input frequencies B+nAC and 11/919. (B+mA-C in device Q causes it to regulate oscillator O to hold? these frequencies equal. 55 or thecondition that B+nA-C=n-/m (B+mA- C), 13 must equal C. This condition C B also establishes the fundamental frequency A for warm.

which there is the greatestrchnnge: ofl indiictanceiof-the C01lS for. a; given small change of: magnetizing; force.

output currents from the tubes 114 and:

will increase or decrease the magnetizationzof the cores ofi'the windings referred: to and change the effective: inductanceof the cir cuit of the associated winding 127. This coil;i2T.is1in-parallcl with the inductance 169 for the frequency-determining circuitof the oscillator; (3 Hence in the case of Fig. 5 as desc ibed thus far.. the adjustment of" 'l'requency: is by adjustment-of the. inducetance insteadof the capacity asiin; Fig: 4;

The adjustment effected by the foregoing methods may be of limited range and a wider but slower. adjustment will be effected" automatically by the differential voltmeter relay 118. As long as the output currents from the two tubes11-1 and 115. are equal, both circuits will be open. at 118, but whenever either current exceeds the other-to a :ertain degree, one of the circuits will be closed correspondingly and one or the other: of theinagnets 131 or 132. will operate the ratchet wheel 119-one way or the other to correspond. long; as the closure of one circuit remains at 118, the slow relay 100 will open and close. thus causing" the ratchet wheel 119to, take progressive steps in the same direction.

T ratchet wheel 119 governs an adjustable condenser 120 which varies the capacity in shunt to the main'condenser 171 of the frequency-determining circuit of the oscillator 0 Hence in the case of Fig, 5, large adjustments are made comparatively slowly by automatic adjustment of the condenser 120. and smaller quicker adjustments are made by varying the effective inductance through the winding 12? in parallel with .the

main inductance 169 of the frequency-deter mining circuit oftheoscillator 0 By opening the switches-99 and throwing the switch 122, the adjustable condenser 120 is thrown out of service and the slow adi stment is effected by means of the rheostat 12.8, whiclrnow becomes a part of the direct current circuit through the coils 124 and 12%. In this case, the slow adjustment. by varying: the direct current through the windings and 12-5 changes the magnetizaiion of their cores to values at which the inductance will be greater or less as required.

dtill another modification for the fre qucncy-determining apparatus is shown in Fig; 6. Here the outputicurrents from. the

7 The condensers 130 serve toclose the circuits of coils 125 and" 125 as seen from the oscillator 0 connected.

Accord two tubes 114 and 115 0 through opposed windings 161 within W ich is an armature 162 on the shaft 164 held at a certain position by the spring 165. The armature 162 is energized by direct current from the battery 163, and hence turns one way or the other when the current in the output circuit of either tube 114 or 115 exceeds the current in the other such circuit.

The coils 166 and 167, with one of them mounted to rotate with the shaft 164, constitute an adjustable industance in shunt to the main inductance 169 of the frequencydetermining circuit of the oscillator 0 Accordingly, the frequency of this circuit is adjusted one way or the other by excess of output current from one tube over the other of the two tubes 114 and 115.

By opening the switches 168 and closing the switches 170, adjustment of the frequency of the oscillating circuit of the oscillator 0 may be effected by adjustment of the capacity of the condenser whose plates are 172 and 173, and which is connected in parallel with the main condenser 171 of the frequency-determining circuit.

The intensity of the received currents in the receiving loop 42 (Fig. 2) will vary over a wide range from time to time according to common experience in radio receiving. But it is important that the intensity of the input currents to the receiving networks R R etc. shall be substantially constant; this is especially requisite for the proper operation of the marginal relays such as NR, and NR of Fig. 3. \Vhile radio transmission may be subject to necessity for adjustment to keep the proper transmission level at the receiving end, on the other hand, it will generally be unnecessary to provide adjustment for phase distortion, because all frequencies are transmitted through the ether with the same velocity.

From the output of signal compensator X a conductor pair 99 conducts received current of frequency mA to an amplifier and the detector 140. In the output circuit of this detector 140 is the polarized relay 103 which is adjusted so that at a certain desired strength of output current, its arma ture will be open between two opposed contacts, but a slight increase of current strength will close it on one contact and a slight decrease will close it on the other contact. Such closure either way will cause motor M to rotate in a corresponding direction and adjust the artificial line AL to compensate the increased or decreased intensity and restore the armature of relay 103 to its neutral position.

The adjustment effected as described above by means of relay 103 may be somewhat slow or delayed and a supplemental quick adjustment over a narrow range is afforded by means now to be described.

Referring again to Fig. 2, the plate voltage for the amplifier 129 is supplied from the negative end of the resistance 144 in the plate circuit of the detector 140. Any change in the received transmission level on conductors 99 results in a corresponding change in the output current of the detector 140, as already explained. Due to the resistance 144, this change in detector current will vary the plate voltage on amplifier 129 in the opposite direction and change the gain of the amplifier accordingly, thus tending to maintain the level delivered to the busbars 102 constant. An alternative method of controlling the gain of amplifier 129 may be used by closing the switch 150 and throwing switch 145 to its other position. The plate volt-age on amplifier 129 is now supplied by the battery 201 and is constant. However, the effective bias grid voltage applied to amplifier 129 will now vary with the output current in detector 140, inasmuch as the current from the negative side of 144 through the resistances to ground on the filament of 129 will vary; e. g., an increase in output current of 140 will give an increased negative voltage on the grid of 129 and so reduce the gain of the amplifier. It is evident that this change in gain of the amplifier is the same as that in the alternative case described first.

The receiving network such as R, in Figs. 2 or 3 connected to the input conductors 108 may be replaced by the apparatus shown in Fig. 7. The local generator 6:, of the current of low frequency mA is the same as for Figs. 2 and 3, and the current of this frequency is split in two components 90 apart as in those figures. One of these two components is applied through transformer 180 to the terminals of the carbon button 181. The incoming signal currents on conductors 108 go through a magnet winding that varies the pressure on this carbon button 181, and varies its resistance correspondingly. It will be remembered that this incoming current has two components 90 apart, each modified by amplitude changes and by phase reversals for signals. With proper adjustment of the phase shifter P one such component is either in phase or at 180 with the applied electromotive force through button 181 and the other such component is in quadrature. The variation of resistance of the component of incoming current in phase is shown in Fig. 8. The resultant current component, being determined by electromotive force divided by resistance, is readily plotted. Its positive half wave is obviously of much greater value than its negative half wave and the average resultant current has a substantial positive value as shown by the appropriately labeled horizontal dotted line. But when the incoming current component is at 180 with the locally applied electromovalue.

The direct current relays 182 and 183 of Fig. 7 will not be influenced by the current of Fig. 9 but they will be influenced by the current of Fig. 8 if it be of sufficient magnitude. The relay 182 is a polarized relay and shifts its armature when the current of Fig. 8 changes from one dotted line value to the other, that is, when the incoming current changes 180 in its relation to the electromotive force applied through the button. On the other hand, the relay 183 is neutral and marginal and operates only for more pronounced variation of the resistance through the carbon button 181,, that is, it

' operates only for strong currents in the windings suppl'ied'by conductors 108 as determined by closure ofkey K at the sending nd. Each relay 182 or 183 controls a circuit at or 92, respectively, for the signal compensator X as in Fig. 3.

Juvt as relays 182 and 183 operate on one 90 component of incoming current on conductors 108. so the other pair'ofrelays at the bottom of Fig. 7 operate on the other 90 component of incoming current.

I claim:

1. In a carrier current signal receiving system, a demodulator, a generator to supply current of carrier frequency to said demodulator, frequency controlling means for said generator, and means to compare two different harmonic currents in the modulator output and to actuate the said controlling means thereby.

2. In a carrier current signal receiving sys tem, a demodulator, a generator to supply current of carrier frequency to said demodulator, frequency controlling means for said generator, means to separate two components of different freqcncies in the demodulator output, means to derive therefrom two currents that will be equal in frequency when and only when the said generator operates at the correct frequency, and means to compare said derived currents and by their departure from equality of frequency to make compensating adjustment through said frequency controlling means.

3. In carrier current signaling, the method of synchronizing a current of carrier frequency generated locally at the receiving end for demodulation, which consists in separating out two harmonic currents in the demodulation product, multiplying each by the harmonic number of the other so as to get currents of equal frequency provided the locally generated current is'of the propor frequency, andapplying the 'currents'so obtained to make compensating adjustment of said locally generated current by their departure from the equality relation if the locally generated current is not of the prop er frequency and phase.

4. In a carrier current signal receiving system, a demodulator, a generator to supply current of carrier frequency to said demodulator, frequency controlling means f0r=-said generator, means to :separate :two harmonic components of different 'frequencie's iwthe demodulator output,'means to multiply-each by the harmonic number of the otherto get product currents of equal'frequency provided the said generator is operating at'the'correct frequency, and means to actnatesaid controlling means by departure of said product currents from the equality'relation.

5. In a multiplex carrier current signal receiving system,a demodulator, a generator to supply current of carrier frequency to said demodulator, frequency controiling means for said gencrator,-means to separate signalmodified component currents of: different frequencies from the. demodulator output, local generators of'currents'of corresponding frequencies to cooperate'in de'teoting the signals on these component currents, means to synchronize the locahgenerallors by one such component, and meansito actuate said controlling means by 'f'requencycomparison of another such component-and the locally thereto.

6. In multiplex signaling, the metho'd of discriminating between received current components differing in phase, which consists in applying the received currentlto' vary a resistance and applying through the mesistance an electromotive forcein phase with the component to be received, whereby the current through the resistance 'will conrcspond with the desiredcomponent of t'heureceived current.

7. The method of detecting-signals on each of two components of received cunrent 90 apart in phase, which consistszin applying the received current to vary two're'srstances and applying through these resistances respective locally generated electromoti ve forces 90 apart in phase and corresponding to the said components of received current, whereby the currents througlr'saidresistances vary in accordance with the signal chang'es on the respective components.

8. In combination, a microphone, an elecgenerated current corresponding troma net in o erative relation to'it means a P a to apply to one of these elements a "current having signal -modifications on each 'oftwo components thereof differing in .phase,-.and means to apply to the other element an electromotive force corresponding in phase with one such component, whereby the current through said element will correspond to the signal modifications on the corresponding component.

9. In combination, two microphones. respective electroniagnets, a circuit to apply thereto received signal currents each of two components 90 apart in phase and each component affected by signal modifications,

means to apply to the microphones electromotive forces in phase with said components, respectively, and current indicators in series with said microphones to indicate the signals carried by the respective received current components.

In testimony whereof I have signed my name to this specification this 6th day of May, 1926.

HARRY NYQUIST. 

